Historically, many methods have been developed to improve the delivery of drugs to their target organs with greater organ specificity, to allow drug release over longer periods of time, and to provide special release characteristics. These controlled drug release methods generally involved the use of carrier systems to carry the drug to the vicinity of its target organ and then to release it in a predetermined fashion. These prior art carrier systems employed a number of substances such as a variety of lipids and phospholipids (liposomes), biodegradable and non-biodegradable polymers, mechanical devices, magnets, ultrasound, osmotic systems, and many others. These systems, however, suffered from several drawbacks.
For example, none of these systems have the intrinsic ability to attach themselves to target organs long enough to allow for localized drug release over prolonged periods of time. None of these systems aid in the absorption of the drug or drugs contained therein into the blood stream. These systems are also limited in the molecular size of the biologically active substances they can deliver, since higher molecular weight proteins and other macromolecules are difficult to load into and release from these systems or devices. Such systems have further generally involved problems of incompatibility or reactivity of the carrier or vehicle with the drug, the prevailing environment and/or target organ or body.